Method of Raising the pH of Acidic Bodies of Water

ABSTRACT

The invention relates to a method of raising the pH of a body of water having a pH of less than 4.5 by introducing neutralizing agent, the raising of the pH taking place in at least two stages thus: at pH levels below 4.5, a first neutralizing agent having a final conductivity of not more than 100 .mu.S/cm, and, after attainment of a pH of at least 4.5, a second neutralizing agent, having a final conductivity of more than 100 .mu.S/cm, is introduced into the body of water, the final conductivity of the neutralizing agents being defined as the conductivity of an aqueous suspension or solution of neutralizing agent in solution equilibrium at 25.degree. C., having a neutralizing agent content of 0.015% by weight.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/677,451, filed on Jun. 21, 2010, which claims benefit of PCTapplication no. PCT/EP2008/062264, filed on Sep. 15, 2008, which claimsthe benefit of Germany patent application DE 102007043751.1 filed onSep. 13, 2007 and Germany patent application DE 102007057414.4-41 filedon Nov. 27, 2007, which are hereby incorporated by reference in theirentirety.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the total distribution Q3% is shown in FIG. 1 as a functionof the particle size.

FIG. 2 shows the change in conductivities of the neutralizing agentsolutions against time.

The invention relates to a method for raising the pH of acidic bodies ofwater by introducing neutralising agent. In particular, the inventionrelates to a method for raising the pH of residual open-pit mining lakeswith a water volume of more than 500,000 m³.

Regions in which open mining has been carried out, are often affectedfor decades after mining activity has ceased by its consequences. Aftereliminating the general groundwater reduction, the reappearinggroundwater flows through the dumps left behind by the mining. Inparticular due to the pyrite weathering in open mining, these dumps areoften enriched with a high acid potential. This leads to theacidification of residual open-pit mining lakes being produced, which asa consequence often have pH values of less than 3. Even residual mininglakes which have already been flooded and were originally neutral orhave already been neutralised can acidify if groundwater flows in fromacidic dumps.

By flooding with surface water, the acidification can basically becounteracted, as the latter has a certain neutralisation potential. Athigh degrees of acid or with flooding from residual mining lakespartially already filled with acidic ground water, the smallneutralising potential of surface water is, however, not sufficient forneutralisation. As a result, the lakes remain in their acidic state.

It is known to neutralise acidic water at points, for example at theexit from a lake, by liming. The liming is generally carried out here inpit water purification plants (PWPs). The lake itself, however, likewiseremains in its acidic state and is therefore unusable economically andfrom the point of view of tourism.

Moreover, so-called in-lake methods are known, such as are described,for example, in DE 199 612 43. In these methods, power station ashes,which originate from the combustion of brown coal, are re-suspended inorder to neutralise the acid capacity present in the lake. This methodis, however, only usable if old ashes are present in the body of water.Moreover, the activity of the ashes used and the flow through them arecomparatively small.

DE 103 04 009 describes a method for controlling the water quality ofopen acidic bodies of water, in which, using in-lake method, injectorand mixing technology as well as fine-particle neutralising agents and aconsidered distribution technology, a high degree of efficiency of theneutralising agent used is achieved. With a good mixing of theneutralising agent and a good distribution in the lake alone, thedegrees of efficiency in the material conversion cannot be optimised,however.

DE 41 24 073 A1 and WO 02/016272 A1 also describe methods for treatingacidic bodies of water. The application of the methods described therein large quantities of water, as often occur in residual mining lakes,requires a disproportionately high technical or economical outlay,however.

A method for improving the water quality of acidic bodies of water isknown from DE 10 2006 001 920 A1. In this method, in a first treatmentstage, at a low pH, a calcium- or calcium-/magnesium-containing feedmaterial is introduced, and in a second stage at a higher pH, sodiumhydroxide solution is introduced. In this method, high product feedcosts are overall necessary.

The present invention was based on the object of providing a method forraising the pH of bodies of water with a pH of less than 4.5, in whichneutralising agents are used, the base capacity of which is optimallyutilised. In this manner, the costs of treating the body of water areminimised.

This object is achieved according to the invention by a method forraising the pH of a body of water with a pH of less than 4.5 byintroducing neutralising agent, in which the raising of the pH takesplace in at least two stages in such a way that at pH values of lessthan 4.5, a first neutralising agent with a final conductivity of atmost 100 μS/cm and, after reaching a pH of at least 4.5, a secondneutralising agent with a final conductivity of more than 100 μS/cm isintroduced into the body of water, the final conductivity of theneutralising agent being determined as the conductivity of an aqueousneutralising agent suspension or solution in solution equilibrium at 25°C. with a neutralising agent content of 0.015% by weight.

The method according to the invention is distinguished in that theneutralising agents used are selected and introduced as a function ofthe respective actual state of the pH of the acidic body of water to betreated. In particular, at pH values of below 4.5, neutralising agentswith a final conductivity of at most 100 μS/cm are used, preferably ofat most 70 μS/cm, still more preferably of at most 40 μS/cm and inparticular of at most 20 μS/cm, while at pH values of at least 4.5,neutralising agents with a final conductivity of more than 100 μS/cm,preferably of more than 200 μS/cm, still more preferably of more than300 μS/cm, and in particular of more than 500 μS/cm are used.

The term neutralising agent is taken, according to the invention to meana chemical compound, which has a specific basicity and is in a positionto raise the pH of an acidic body of water.

The term final conductivity of the neutralising agent is taken,according to the invention, to mean the conductivity of an aqueousneutralising agent suspension or solution at 25° C. with a neutralisingagent content of 0.015% by weight, which is in solution equilibrium.

If a chemical compound is placed in the water, as a function of factorssuch as solution enthalpy, particle size, temperature etc, a definedfraction of the compound goes into solution. As a result, theconductivity of the solution is increased. After a certain time, asolution equilibrium is adjusted. This equilibrium is characterised inthat per time unit, the same particle number goes into solution as isprecipitated from the solution. If the solution equilibrium has beenestablished, no further increase in the conductivity of the solutioninvestigated takes place. According to the invention, the finalconductivity is reached if the conductivity does not change by more than10 μS/cm in one minute. The conductivity achieved in the state ofequilibrium therefore represents the final conductivity according to theinvention of the solution under consideration in the conditionsselected.

To define the final conductivity according to the invention, thefollowing conditions were selected. The starting point is to beneutralising agent suspensions with a mass concentration of 1 g solid in100 ml sample volume. From these suspensions, 1.5 ml is placed indeionised water (100 ml) with the temperature controlled at 25° C. andthe adjustment of the solution equilibrium is awaited. The suspensionscontained here have a neutralising agent content of 0.015% by weight.The suspension constituents may be stirred to accelerate the adjustmentof the equilibrium. The solution equilibrium is characterised in thatthe measured conductivity does not change by more than 10 μS/cm in oneminute. The conductivity measured at this instant under the conditionsgiven above, represents the final conductivity of the neutralisingagent, as it is to be understood according to the invention.

It was surprisingly found that neutralising agents with low finalconductivities at low pH values achieve significantly higher effectsthan at high pH values. In particular it was found that neutralisingagents with a final conductivity of at most 100 μS/cm at pH values ofbelow 4.5 have a significantly higher degree of efficiency than athigher pH values. This fact is used by the method according to theinvention in that it consists of at least two stages, with neutralisingagents with comparatively low final conductivities being used in thefirst stage at low pH values. This procedure is advantageous asneutralising agents with low final conductivities are generally moreeconomical than neutralising agents with higher final conductivities.Only at pH values of at least 4.5, in which the reactivity of theneutralising agents with low final conductivities decreases are the moreexpensive neutralising agents with higher final conductivities used.

According to the invention the fact is therefore utilised that until apH of 4.5 is reached, economical neutralising agents are used, whichhave a surprisingly high degree of efficiency at these pH values. On theother hand, more expensive neutralising agents with a higher activityare only used at higher pH values. Overall, the method according to theinvention thus proves to be effective and nevertheless cost-efficient.

The method according to the invention is particularly economical whentreating acidic bodies of water with a high water volume, such as, forexample a water volume of more than 500,000 m³, as large quantities ofneutralising agent are used here.

The pH of the body of water to be treated can be raised to variousvalues by the method according to the invention. However, the pH ispreferably raised to a value of at least 5, in particular of at least 6.

The method according to the invention may also be carried out by theaddition of the neutralising agent to the body of water as a whole(in-lake method) or else at points, for example at the exit of the lake.However, implementation as an in-lake method is particularly suitable,as the effectiveness and cost efficiency of the method according to theinvention is utilised particularly well in this manner.

The neutralising agent may be introduced in the most varied manners intothe body of water. Excellent results are achieved if the neutralisingagent is introduced in the form of a suspension, in particular anaqueous suspension, into the body of water.

Practical series of tests were carried out, in which the degree ofefficiency of the neutralising agents used was investigated insuspensions of different concentration while varying the pH. It wassurprisingly found here that at pH values of more than 4.5, the degreeof efficiency of the neutralising agents drops, if the suspensioncontains more than 2% by weight of neutralising agent. This drop in thedegree of efficiency is connected with an increase in the product feedquantity and therefore with rising costs.

For this reason, it is particularly preferred according to the inventionif the raising of the pH takes place in such a way that the neutralisingagent is introduced at pH values of below 4.5 in the form of asuspension with a concentration of 2 to 15% by weight, preferably from 5to 10% by weight, of neutralising agent and at pH values of at least 4.5in the form of a suspension with a concentration of 0.05 to 2% byweight, preferably from 0.1 to 1% by weight, of neutralising agent. Inthis embodiment, the neutralising potential present of the neutralisingagent used is optimally utilised as a function of the actual state ofthe pH of the body of water to be treated. This leads to a furtherincrease in the efficiency of the method according to the invention.

At a pH of below 4.5, the most varied materials can be used according tothe invention as the neutralising agents as long as these materials arealkaline and have a final conductivity of at most 100 μS/cm. Accordingto the invention, materials originating from lime are preferably used,specifically, in particular, chalk, lime, limestone slurry, carbokalk,half-burnt dolomite, dolomite grit and/or dolomite rock meal. Thesematerials are preferred according to the invention as they areparticularly economical.

At a pH of at least 4.5, the most varied materials can also be usedaccording to the invention as neutralising agents as long as they arealkaline and have a final conductivity of at least 100 μS/cm. Accordingto the invention materials originating from lime are preferably used,specifically, in particular, burnt lime, lime hydrate, slaked burntlime, dolomite grit, dolomite burnt lime and/or dolomite lime hydrate.These materials are preferred at pH values of at least 4.5 as they havehigh activity.

Apart from the final conductivity, the solution affinity of theneutralising agent used also plays an important part. It is particularlyadvantageous if, at low pH values, neutralising agents with a lowsolution affinity are used. These neutralising agents are generally moreeconomical than those with a high solution affinity. Thus, thisembodiment is particularly cost-efficient. At higher pH values, it isadvantageous, on the other hand, to use neutralising agents with highersolution affinities. The dissolution speed of alkaline compounds namelygenerally decreases with an increasing pH. Thus, when using neutralisingagents which originally have a small solution affinity, there is a riskat higher pH values of these sinking to the bottom of the body of waterwithout having completely reacted and thus being removed from theneutralising process.

Practical tests have shown that the method according to the invention isparticularly effective if at a pH of the body of water of below 4.5,neutralising agents are used, which have a solution affinity, measuredas a sulphuric acid consumption in a pH stationary titration of 0.5 gneutralising agent in 100 ml deionised water at 20° C., by means of 0.5mol/l sulphuric acid at a pH of 6 and for a period of 30 minutes, ofless than 6.5 ml, preferably of less than 5 ml, and in particular ofless than 2 ml.

pH stationary titration is a standard method for determining theneutralisation kinetics of alkaline substances. The substance, thesolution affinity of which is to be determined, is prepared at roomtemperature as a suspension while stirring. An acid is now addeddrop-wise to this suspension at a speed such that the pH adjustsstationarily to a predetermined pH. After identical time periods, ineach case, compounds with a higher solution affinity have a higher acidconsumption than those with a lower solution affinity. This is to beattributed to the fact that in compounds with a higher solution affinityper time unit, a larger quantity of base goes into solution and isavailable there for neutralising the acid added drop-wise.

The solution affinity, as it is to be understood in the sense of themethod according to the invention, relates to the consumption in ml of0.5 mol/l sulphuric acid, which can be introduced into a suspension of0.5 g neutralising agent in 100 ml deionised water in a 250 ml beaker(wide) within 30 minutes, without the pH of the suspension falling below6, while the suspension is stirred at 20° (±2° C.) with a magneticstirrer and a magnetic stir bar of about 30 mm in length and about 7 mmin diameter at a speed of 800 rpm. According to the preferred embodimentof the invention mentioned above, at pH values of the body of water ofless than 4.5, neutralising agents are preferably used of the type wherethe solution affinity, measured as the sulphuric acid consumption by themethod described above, is less than 6.5 ml, preferably less than 5 mland, in particular, less than 2 ml.

At pH values of the body of water of at least 4.5, on the other hand,neutralising agents are preferably used of the type where the solutionaffinity, measured as the sulphuric acid consumption by the methoddescribed above, is at least 6.5 ml, preferably more than 8 ml and, inparticular, more than 10 ml.

According to a further preferred embodiment of the invention, at pHvalues of the body of water of less than 4.5, materials originating fromlime with a particle size distribution D50 of more than 7.4 μm,preferably more than 9, and in particular more than 11 μm are used asthe neutralising agent. Materials originating from lime with a particlesize distribution of this type are characterised in that half of theparticles contained in them have a diameter of less than 7.4. Because ofthe larger surface, materials with a smaller D50 value have a higherreaction capacity and therefore a higher solution affinity thanmaterials of the same type, which have a higher D50 value.

Materials originating from lime with a particles size distribution D50of less than 7.4 μm, preferably of less than 5 μm and, in particular, ofless than 3 μm, are accordingly particularly suitable neutralisingagents according to the invention at pH values of the body of water ofat least 4.5.

The invention will be described in more detail below with the aid ofthree embodiments.

Table 1 show the analyses of the neutralising agents used in theembodiments.

TABLE 1 Dolomite Lime Lime Dolomite Half- Lime Dolomite lime milk 1 milk2 lime burnt Dolomite Dolomite Powdered hydrate lime 1 hydrate 1 (20%)(45%) hydrate 2 Ash dolomite rock meal lime 2 Chalk limestone CaO 72.853.09 45.01 73.5 72.9 42.66 49.57 38.87 29.75 60.00 50.43 54.69 MgO 0.7438.92 31.81 0.6 0.6 30.68 1.91 26.08 21.5 32.74 0.45 0.50 SiO₂ 3.30 1.781.65 0.4 7.17 10.6 0.08 0.77 2.97 6.36 0.85 SO₃ 0.16 0.17 0.37 0.2 0.1225.65 0.01 0.03 0.53 0.09 0.047 Fe₂O₃ 0.33 0.82 0.93 0.1 3.42 4.00 0.280.2 0.89 0.39 0.11 Al₂O₃ 0.25 0.70 0.78 0.1 2.8 4.95 0.05 0.30 0.94 1.360.21 Mn₂O₃ 0.03 0.14 0.21 0.24 0.06 0.11 0.06 0.03 0.04 0.04 K₂O 0.080.06 0.02 0.62 0.15 0.01 0.02 0.12 0.21 0.06 Cl n.k. 0.06 0.08 0.02 0.070.02 0.01 0.05 0.00 n.k. CO₂ 2.36 3.98 1.47 3.27 n.k. 33.72 n.k. 0.7840.08 43.48 (calculated) (calculated) GLV 22.11 4.24 19.04 24.8 25.312.27 3.02 34.38 47.16 1.70 40.53 n.k. Total 99.8 99.98 99.90 74.9 73.5100.00 99.98 99.89 99.86 100.00 99.86 99.99

Table 2 shows the particle size distribution of the neutralising agentsused in the embodiments

TABLE 2 Particle size Sv in μm D10% D50% D90% D97% D100% m2/cm3 Limemilk 1 0.9 2.6 6.7 9.9 21.0 3.1 (20%) Lime milk 2 0.9 2.8 8.5 14.6 51.02.9 (45%) Lime hydrate 1.1 6.7 56.6 82.5 123.0 1.9 Lime milk 1.8 7.634.5 102.1 206.0 1.3 dolomite lime 1 (30%) Dolomite 1.5 7.4 27.5 51.5103.0 1.6 lime hydrate 1 Dolomite 1.2 8.5 72.2 98.9 147.0 1.7 limehydrate 2 ground Ash 2.5 20.3 97.7 135.1 206.0 0.9 Half-burnt 47.5 174.2314.3 379.9 515.0 0.1 dolomite Dolomite 2.7 16.2 102.1 163.9 246.0 0.9rock meal Dolomite 2.1 17.4 60.6 80.5 103.0 1.1 lime 2 Chalk 0.8 2.6 7.612.1 30.0 3.1 Powdered 1.5 8.9 28.4 47.8 103.0 1.5 limestone

The total distribution Q3% is shown in FIG. 1 as a function of theparticle size.

EXAMPLE 1 Determination of the Conductivity of Lime Milk and LimeHydrate

Purpose and Application Area

The method is used to determine the conductivity of alkaline compoundssuch as lime milk and lime hydrate. It is used, in particular, todetermine the final conductivity and the reactivity (dissolution speed)of lime milk products such as lime hydrates.

2. Basis of the Method

The reactivity of lime milk and lime hydrate may expediently be definedby the dissolution speed thereof in water. This is accessible byconductometric methods. The reactivity test described below is based onthe change rapidly taking place in the conductivity after metering alime milk or a lime hydrate suspension as a result of the increase inthe ion concentration, caused by the dissolution of calcium hydroxide.As a result, the final conductivities according to the invention ofalkaline substances such as lime milk and lime hydrate are obtained aswell as times, by which characteristic fractions of the solid have goneinto solution.

3. Equipment

-   -   3.1 Conductometer with rigidly adjustable measuring range (for        example 0-2 ms/cm)    -   3.2 Conductivity measuring cell with non-sheathed and        non-platinised Pt-electrodes.    -   The pre-treatment of the electrode should be carried out in        accordance with the specifications of the equipment producer.    -   3.3 Computer with suitable software for data collection.    -   3.4 150 ml measuring vessel with thermostat casing and with a        covering, which is provided with openings for the measuring        cell, the propeller stirrer, the gas feed, the gas outlet and        the sample feed.    -   3.5 Thermostat.    -   3.6 Propeller stirrer or suitable magnetic stirrer.    -   3.7 Pipette (1.5 ml) with metering device.

4. Reagents

-   -   4.1 Potassium chloride solution, c(KCl)=0.01 mol/l.    -   4.2 Potassium chloride solution, ω(KCl)=3%.    -   4.3 Water, deionised, CO₂-free.    -   4.4 Nitrogen, N₂.

5. Measuring Method

5.1 Measuring Process

5.1.1 Preparation of the Apparatus

As the freely rinsable, but not sheathed plate electrodes change theircharacteristics after repeated use, at the beginning of each measuringseries, the cell constant of the electrode should be determined withpotassium chloride solution (4.1). The electrode should be placed in themeasuring vessel in such a way that the electrode faces are parallel tothe movement direction of the water.

As the dissolution speed of rapidly soluble lime milks or lime hydratesdepends on various factors, such as the stirrer type, stirrer speed,vessel dimensions and position of the measuring cell and meteringlocation, it is sensible to define the measuring apparatus by means ofmeasuring performance data (homogenisation time).

5.1.2 Determination of the Homogenisation Time

100 ml water (4.3) are prepared in the sample vessel and the temperatureis controlled to 25° C. The vessel is rinsed with nitrogen during themeasurement in order to prevent a retrospective take up of CO₂. With thestirrer running, which should run at as high a speed as possible, butwithout too strong a vortex formation, and after the starting of therecording of the measured value, 1.5 ml KCl solution (4.2) is meteredwith the pipette (3.7) into the prepared water.

5.1.3 Determination of the Dissolution Speed

At the beginning, with lime milk samples, the dry substance should bedetermined, as, for the measurement, the suspension is adjusted with amass concentration of 1 g solid in 100 ml sample volume. To produce thissuspension, a volume equivalent to this solid content of the lime milkis removed and filled to 100 ml. A suspension is also produced from limehydrates with a mass concentration of 1 g/100 ml. The samples are leftto rest for about 30 minutes (complete wetting of the hydrate surface).1.5 ml of the sample are metered as fast as possible into the water(4.3) (100 ml) with the temperature controlled to 25° C. in the samplevessel.

5.2 Evaluation

5.2.1 Establishing the Starting Point t=0 and Defining the FinalConductivity

The measured conductivity change is recorded as a function of the time.The curve obtained here has to be recorded until the maximumconductivity is reached. For highly reactive finely dispersed lime milk,a recording of the measured value over two minutes is generallysufficient, a value being recorded every 0.1 s. The starting time (t=0)is established for the first measured value, at which the conductivitychanges by more than 20 μS/(cm −0.1 s).

The final conductivity ae_(max) is reached when the conductivity changesby no more than 10 μS/cm in one minute. ae_(max) is calculated as theaverage from the measured values from reaching the final conductivityover a time interval of 1 min.

The homogenisation time (final conductivity ae_(max) of the KCl solution(4.2)) should be <2.5 s.

5.2.2 Reading Off the Dissolving Times and Information on the Results

To measure the lime samples, the dissolving times are given in secondsfor which 63, 80, 90 and 95% of the maximum conductivity are reached.The conductivities ae (x %) are calculated according to the followingequation.

ae(x %)=ae _(max)/100

The corresponding dissolving times t (x %) are read off from theconductivity time curves.

6. Annex

Measuring devices other than those described under section 3 can also beused. It is must be ensured here that the homogenisation timeestablished in section 5.2.1 is adhered to. To determine thehomogenisation time, the metering volume has to be selected such thatthe final conductivity reaches (900±50) μS/cm. This metering volume hasto be retained to measure the samples.

EXAMPLE 2 Determining the Final Conductivities of the NeutralisingAgents Investigated

In order to determine the final conductivities of the neutralisingagents investigated, the process is as described under Example 1.

The results are shown in FIG. 2 and Table 3. It is shown that theconductivities of the neutralising agent solutions investigated firstlyincrease sharply and then tangentially approach a final conductivityvalue. The neutralising agents limestone meal, half-burnt dolomite,dolomite rock meal and chalk have final conductivities of below 100μS/cm.

TABLE 3 LM Half- Conversion LM 1 LM 2 DML 1 DMLH DMLH DMLH DMLH 2 burntDML after s 20% 45% LH 30% 1 2a 2b ground Ash dolomite DMRM 2 Chalk PL63% 0.8 0.9 3.0 2.9 1.7 2.9 0.5 1.5 7.2 n.k. n.k. 9.0 n.k. n.k. 80% 1.11.5 14.1 9.4 3.6 70.8 71.2 17.5 50.4 n.k. n.k. 27.9 n.k. n.k. 90% 1.52.7 34.6 19.3 7.9 208.5 191.9 121.4 160.1 n.k. n.k. 74.0 n.k. n.k. 95%1.9 4.5 59.7 32.1 16.9 418.9 330.5 403.8 352.8 n.k. n.k. 172.2 n.k. n.k.100% 7.8 38.0 285.1 392.8 116.1 1189.4 1085.0 1198.2 1149.9 >1800 >18001781.0 n.k. n.k. Final 928 939 787 540 512 234 202 356 335 54 10 717 3232 guide value [μS/cm]

EXAMPLE 3 Determination of the Solution Affinity of the NeutralisingAgent by pH Stationary Titration

In Example 3, the neutralising kinetics of the different neutralisingagents is determined in the acidic range by pH value stationarytitration with sulphuric acid at a pH of 6. 0.5 g of the respectiveneutralising agent are prepared in 100 ml deionised water in a 250 mlbeaker (wide) at 25° C. with stirring with a magnetic stirrer and amagnetic stir bar of 30 mm in length and about 7 mm in diameter at astirring speed of 800 rpm and 0.5 molar sulphuric acid added dropwise ata speed such that the pH is stationarily adjusted to 6. The quantity ofsulphuric acid, which can be added within 30 minutes without a pH of 6being fallen below is established and supplies a measure of the solutionaffinity of the neutralising agents investigated in the acidic medium.

The theoretical consumption of sulphuric acid for the reaction:

H₂SO₄+2Ca(OH)₂→CaSO₄+2H₂O

is 13.7 ml.

Table 4 shows the consumption of sulphuric acid in ml after 30 minutesfor the neutralising agents used. It is shown that the various productshave a different sulphuric acid consumption. Neutralising agents with ahigh reactivity lead to an overall higher sulphuric acid consumptionthan products with lower reactivity.

TABLE 4 pH Consumption starting H₂SO₄ in ml Product value after 30 min 1Lime milk 1 (20%) 12.6 13.7 2 Lime milk 2 (45%) 12.5 13.6 3 Lime hydrate12.6 12.2 4 Lime milk dolomite lime 1 12.6 14.0 (30%) 5 Dolomite limehydrate 1 12.6 13.5 6 Dolomite lime hydrate 2a 12.3 6.8 7 Dolomite limehydrate 2b 12.3 6.5 8 Dolomite lime hydrate 2 12.3 9.1 ground 9 Ash 12.34.5 10 NaOH 50% 13.0 12.7 11 Half-burnt dolomite 11.0 2.1 12 Dolomiterock meal 9.6 0.7 13 Dolomite lime 12.2 8.3 14 Chalk 9.9 7.6 15 Powderedlimestone 9.8 4.3

The invention claimed is:
 1. A method for raising the pH of a body ofwater, wherein said body of water has a pH of less than 4.5 comprisingintroducing a neutralising agent, such that raising the pH takes placein at least two stages; wherein at pH values of less than 4.5, a firstneutralising agent with a final conductivity of at most 100 μS/cm isintroduced into the water and, after the body of water reaches a pH ofat least 4.5, a second neutralising agent comprising materialsoriginating from lime with a final conductivity of more than 100 μS/cmis introduced into the body of water, wherein the final conductivity ofthe neutralising agents being determined as the conductivity of anaqueous neutralising agent suspension or solution in solutionequilibrium at 25° C. with a neutralising agent content of 0.015% byweight introduced into the body of water at pH values of less than 4.5as a suspension at a concentration of 2 to 15% by weight and, when thepH values are at least 4.5, said neutralising agent is introduced intothe body of water as a suspension at a concentration of 0.05 to 2% byweight.
 2. The method according to claim 1, wherein the pH of a body ofwater with a water volume of more than 500,000 m³ is raised.
 3. Themethod according to claim 2, wherein the pH of the body of water israised to a value of at least 5, preferably to at least
 6. 4. The methodaccording to claim 3, wherein the method is carried out as an in-lakemethod.
 5. The method according to claim 4, wherein the neutralisingagent is introduced as a suspension into the body of water.
 6. Themethod according to claim 5, wherein the materials originate from lime,preferably chalk, lime, limestone slurry, carbokalk, half-burntdolomite, and/or dolomite rock meal, are used as the neutralising agentwith a final conductivity of at most 100 μS/cm.
 7. The method accordingto claim 6, wherein burnt lime, lime hydrate, slaked burnt lime,dolomite burnt lime and/or dolomite lime hydrate is used as theneutralising agent with a final conductivity of more than 100 μS/cm. 8.The method according to claim 7, wherein at pH values of the body ofwater of less than 4.5, a neutralising agent is used with a solutionaffinity, measured as the sulphuric acid consumption in a pH stationarytitration of 0.5 g neutralising agent in 100 ml deionised water at 20°C., by means of 0.5 mol/l sulphuric acid at a pH of 6 and for a periodof 30 minutes, of less than 6.5 ml, preferably of less than 5 ml and, inparticular, of less than 2 ml.
 9. The method according to claim 8,wherein at pH values of the body of water of at least 4.5, aneutralising agent is used with a solution affinity, measured as thesulphuric acid consumption in a pH stationary titration of 0.5 gneutralising agent in 100 ml deionised water at 20° C., by means of 0.5mol/l sulphuric acid at a pH of 6 and for a period of 30 minutes, ofmore than 6.5 ml, preferably of more than 8 ml and, in particular, ofmore than 10 ml.
 10. The method according to claim 9, wherein at pHvalues of the body of water of less than 4.5, materials originating fromlime with a particle size distribution D50 of at least 7.4 μm,preferably of more than 9 μm, and in particular of more than 11 μm, areused.
 11. The method according to claim 10, wherein at pH values of thebody of water of at least 4.5, materials originating from lime with aparticle size distribution D50 of less than 7.4 μm, preferably of lessthan 5 μm, and in particular of less than 3 μm are used.
 12. The methodaccording to claim 1, wherein the pH of the body of water is raised to avalue of at least 5, preferably to at least
 6. 13. The method accordingto claim 1, wherein the method is carried out as an in-lake method. 14.The method according to claim 1, wherein the neutralising agent isintroduced a suspension into the body of water.
 15. The method accordingto claim 1, wherein the materials originate from lime, preferably chalk,lime, limestone slurry, carbokalk, half-burnt dolomite, and/or dolomiterock meal, are used as the neutralising agent with a final conductivityof at most 100 μS/cm.
 16. The method according to claim 1, wherein burntlime, lime hydrate, slaked burnt lime, dolomite burnt lime and/ordolomite lime hydrate is used as the neutralising agent with a finalconductivity of more than 100 μS/cm.
 17. The method according to claim1, wherein at pH values of the body of water of less than 4.5, aneutralising agent is used with a solution affinity, measured as thesulphuric acid consumption in a pH stationary titration of 0.5 gneutralising agent in 100 ml deionised water at 20° C., by means of 0.5mol/l sulphuric acid at a pH of 6 and for a period of 30 minutes, ofless than 6.5 ml, preferably of less than 5 ml and, in particular, ofless than 2 ml.
 18. The method according to claim 1, wherein at pHvalues of the body of water of at least 4.5, a neutralising agent isused with a solution affinity, measured as the sulphuric acidconsumption in a pH stationary titration of 0.5 g neutralising agent in100 ml deionised water at 20° C., by means of 0.5 mol/l sulphuric, acidat a pH of 6 and for a period of 30 minutes, of more than 6.5 ml,preferably of more than 8 ml and, in particular, of more than 10 ml. 19.The method according to claim 1, wherein at pH values of the body ofwater of less than 4.5, materials originating from lime with a particlesize distribution D50 of at least 7.4 μm, preferably of more than 9 μm,and in particular of more than 11 μm, are used.
 20. The method accordingto claim 1, wherein at pH values of the body of water of at least 4.5,materials originating from lime with a particle size distribution D50 ofless than 7.4 μm, preferably of less than 5 μm, and in particular ofless than 3 μm are used.